1. Field of the Invention
This invention relates to a high frequency filter having variable filtering characteristics that can be changed in response to the control voltage applied to it.
2. Related Art
As a result the technological development in the field of radio communications and the introduction of new regulatory telecommunication systems in recent years, there is an ever-increasing demand for telecommunication devices that are adapted to a number of transmission/reception systems. For a telecommunication transmission/reception device to adapt itself to a plurality of transmission/reception systems, it needs to be provided with a number of filters that are formed by using a so-called bulk wave resonator such as a dielectric resonator, a surface acoustic wave (SAW) resonator, a crystal oscillator, or an LC resonator, and it needs to have respective pass band characteristics that are different from each other. However, such an arrangement is not appropriate from the viewpoint of downsizing the telecommunication device.
Therefore, development of a high frequency filter having variable filtering characteristics which can be made to change by an external control means has been expected from the viewpoint of reducing the number of filters to be used in such a telecommunication device and thereby down-sizing the device. Proposals that have been made in this technological field include those relating to devices comprising a resonance circuit realized by connecting a voltage control type variable capacitor such as varactor to a piezoelectric thin film resonator in series or in parallel (e.g., D. Penunuri, et al., “A tunable SAW Duplexer”, Proc. IEEEE Ultrasonics Symposium, 2000, pp. 361-366). However, such devices are accompanied by a problem that the resonance frequency depends on the electro-mechanical coupling coefficient k2 of the resonator and hence the variable frequency range is narrow and that the filter characteristics can be degraded due to the low Q value of the varactor.
As pointed out above, the demand for telecommunication devices adapted to a plurality of transmission/reception systems has been increasing and hence there is a strong demand for small high frequency filters having variable filtering characteristics that advantageously can be made to change appropriately.
In view of the above identified circumstances, it is therefore the object of the present invention to provide a novel down-sized high frequency filter having filtering characteristics, including a broad variable frequency range that is variable and can be made to change by an appropriate external control means, and showing a high frequency stability that is not significantly degraded with time.